Powered pipe tongs have been in use for many years primarily to manipulate threaded pipe connections related to pipe suspended in earth boreholes, mostly oil wells. A powered rotating assembly mounted in a stationary frame rotates the pipe. The torque required to turn the pipe is used to force a pipe gripping die, or dies, into contact with the pipe outer surface.
To transfer torque from a motor to the pipe, an outer ring is driven, and cam surfaces on the outer ring engage cam followers that can move radially toward the pipe. The cam surfaces have an angle that defines the ratio of radial cam follower force and peripheral force. Peripheral forces, in this case, are proportional to torque. Cam followers are directly connected to pipe gripping dies. When the system is in equilibrium, the pipe, dies, cam followers, and cam surfaces rotate in unison.
To rotate the resisting pipe, the outer surface must be gripped. To assure gripping, a brake is applied to an assembly rotationally involving the cam followers and hence the dies, and finally the pipe, as the dies grip the pipe. The brake is not released until the die surfaces have imbedded into the pipe. After brake release, torque transferred to the pipe and radial die forces are related by cam angles.
Cam angles have been the subject of considerable study. Evolution has favored a cam angle of about two degrees to imbed the dies. This cam angle, however, can crush the pipe as full torque is applied. Evolution has further favored a continuous increase in the cam angle, as rotation between cam and follower proceeds. The cam angle may increase to the range of twelve degrees to reduce radial die loads at high torque.
The constantly changing cam angle works very well to imbed the die yet protect the pipe as long as the overall dimension system accommodates the intent. The overall dimension system includes the obvious in the tong system, but also includes the pipe outer diameter. There is no problem with different pipe size ranges; the tong components are changed to accept different size ranges. Small diameter differences, from such as tolerance and wear, within one size range is the problem. If the pipe is oversize, the dies will be forced only by the cam low angles and never will be protected by the higher angles. Conversely, if the pipe is undersize, the dies will not be imbedded by the low angles, and the higher cam angles will allow slip. Both problems may damage pipe.
Some system is needed to always assure imbedded dies but never crush pipe.